Conventional methods of determining a location of a mobile device that include the use of time difference of arrival (“TDOA”) have been proposed. However, such methods impose limits on performance and/or fail to capture accurate performance when used within indoor environments. The limitations on performance of such proposed methods are due to limits on signal resolution, transmission signal multipath impact, and limited effective sensitivity given lack of capability to resolve target signals.
Accordingly, such proposed methods are limited to tracking only a few devices at once and may lack ability to continuously track mobiles even though several hundred devices may operate within a targeted interior area of interest. Furthermore, proposed mobile systems that measure the time differences of arrival among surrounding mobile uplink signals require special Location Measurement Unit (“LMU”) receivers. Such LMU receivers are installed as an “overlay” to existing communication networks. That is, the LMU receivers operate as a separate network of widely distributed receivers located on cell towers or rooftops that are separate from the communication network to which the mobile device belongs. These cellular network LMUs are designed to detect and demodulate individual mobile devices operating across relatively wide, outdoor geographic areas. These receiver devices are relatively expensive to produce and maintain, and are only approaching economic practicality where they cover relatively large outdoor coverage areas covering potentially thousands of mobiles and are primarily motivated by compliance with mobile E911 mandates or for relatively special national security areas.
The Third Generation Partnership Project (“3GPP”) standards body has developed standards for forthcoming releases of the Long Term Evolution protocol (“LTE”) that provide a specially embedded Position Reference Signal (“PRS”) on the downlink and Sounding Reference Signal (“SRS”) on the uplink which along with other features are purported to enable high precision TDOA measurements. However, the 3GPP LTE standard using PRS and SRS does not prescribe a particular method of PRS or SRS processing, detection, system configuration, or method of measurement. The standard only requires the mobile and base station generate the PRS or SRS in a prescribed fashion to allow different vendors' innovations. Also it's important to note that the presence of PRS or SRS signals do not upset compatibility with earlier LTE standard released equipment. Earlier LTE standards-based mobiles may therefore be located using the same method but with less precision than the later releases of the protocol. Similar uplink and downlink signal range measurement methods exist for later version WiFi 802.11 standards.
The motive for long range path measurements is to estimate mobile device location, and historically have been developed using either base to mobile (downlink) or mobile to base station (uplink) communication signals as a physical path sounding resource. Wide-area, macrocellular techniques are established, but in most cases mobile operators or their clients rely primarily on alternative position determination methods, especially those that are GPS based. These methods are useful but are also imprecise. For instance, GPS and assisted-GPS (A-GPS) may be useful outdoors or in lightly obscured environments. A-GPS has been successfully augmented using alternative Global Navigation Satellite Services (GNSS) constellations and associated signals, or are combined with less precise terrestrial Cell ID, WiFi or other macrocellular proximity or range measurements. However, these method still lack sufficient accuracy, resolution or coverage indoor not to mention are slow in terms of time to fix for useful mobile position measurements inside typical buildings.
Mobile cellular “hybrid” position measurements (these include some or all of cell ID, cell sector, A-FLT or database retrieval of estimated access point positions based on geo-tagged or crowd-sourced WiFi beacons) uncertainties normally exceed the size of most building floors, thus providing an insufficient level of accuracy to locate a mobile device in a building. These methods also lose effectiveness where there are many mobiles to be located at once or require sub-second mobile location updates (or continuous location tracking).